aaudio: apply volume in the MMAP data path

The volume scaling is in AudioStreamInternal and not the mixer
because we will need volume scaling in EXCLUSIVE mode too.

Bug: 37518243
Test: play a tone using NativeOboe app then press volume keys
Change-Id: Ibbac9770ea4493f8ade64681be86f109a92803cd
Signed-off-by: Phil Burk <philburk@google.com>